Split autoformatters and linters into different workflows and CI jobs

Status quo

Currently we mix formatters and linters with pre-commit:

formatters

https://github.com/pytorch/vision/blob/578c1546c328c68e601bfd2e2309ed175ee343d9/.pre-commit-config.yaml#L9-L11

https://github.com/pytorch/vision/blob/578c1546c328c68e601bfd2e2309ed175ee343d9/.pre-commit-config.yaml#L23-L26

linters

https://github.com/pytorch/vision/blob/578c1546c328c68e601bfd2e2309ed175ee343d9/.pre-commit-config.yaml#L5-L8

https://github.com/pytorch/vision/blob/578c1546c328c68e601bfd2e2309ed175ee343d9/.pre-commit-config.yaml#L31-L32

https://github.com/pytorch/vision/blob/578c1546c328c68e601bfd2e2309ed175ee343d9/.pre-commit-config.yaml#L37

In addition we have a separate CI job that lints with mypy

https://github.com/pytorch/vision/blob/578c1546c328c68e601bfd2e2309ed175ee343d9/.circleci/config.yml#L298

Proposal

I propose we change the rationale from the two CI jobs from pre-commit and mypy to code format and lint. That means, we would only keep autoformatters in our pre-commit configuration and move all linters to what is currently the mypy job.

Pros

• We could use pre-commit as "single source of truth" for formatting code. Currently they mentioned as "purely optional" in our contribution guide. Since pre-commit supports running the hooks manually, we can simply treat it as our way to bundle all autoformatters while the user does not need to know or care what exactly is run.
• We could simply add new autoformatters if they are available through pre-commit, e.g. #5158. If anyone applies code formatting through pre-commit anyway, that won't break any workflow.

Cons

• We would loose the ability to run linters in a bundled manner. I don't think this is a strong con, since linters such as flake8 or pydocstyle trigger very seldom anyway due to the auto-formatting. Plus, we already need to run mypy separately.

cc @seemethere

Start by comparing the formatter and linter hooks in .pre-commit-config.yaml with the mypy job and related CI configuration in .circleci/config.yml. Separate the formatting and linting responsibilities so the resulting CI jobs and pre-commit setup match the proposal, then verify the contribution guidance reflects the new roles.